This invention relates to a developing device for a xerographic copying machine, and more particularly to a magnetic brush developing device therefor.
A magnetic brush developing device is used to develop an electrostatic latent image formed on the surface of a photosensitive drum. The magnetic brush is formed by the bristle of a developer composed of electroconductive toner on a magnetic brush roll containing a magnet therein or an insulating toner on an electroconductive carrier.
Since the surface speed of the photosensitive drum is increased and accordingly the rotating speed of the magnetic brush roll is increased as the copying speed of the xerographic copying machine is accelerated, more toner lumps are produced in the developing housing of the developing device. The toner lumps tend to ride on an accelerated entrained air stream adjacent the peripheral surface of the photosensitive drum and are eventually discharged out of the housing in great amounts. This introduces various problems in the machine including a reduction in the performance of various components, a deterioration of the quality of the copying operation due to the contamination of components in the copying machine and also requires an enlargement of the exhaust system of the machine all of which makes maintenance of the machine difficult.
There have been proposed various ways to remove such difficulties of the conventional magnetic brush developing device. For example, it has been proposed that such a magnetic brush developing device be constructed with a large number of slits or air pockets at an opening of the developing housing in the region where an entrained air stream is produced at the peripheral surface of the photosensitive drum to decelerate the flow velocity of the air stream above the peripheral surface of the photosensitive drum thereby causing toner lumps moving with the air stream to drop in the housing. However, the toner lumps cannot be completely prevented from flowing out of the housing by such slits or air pockets. That is, this approach does not have sufficient capability to prevent the toner from being leaked from the device.
FIG. 1 shows another approach in which the conventional magnetic brush developing device operates to carry a developer c onto the surface of a photosensitive drum d. The magnetic force of a magnetic brush roll b provided in a developing housing a forms a bristle e in the developer c, and this bristle e is called a "magnetic brush". An electrostatic latent image on the surface of the drum d is developed by the magnetic brush e. Reference character f represents an agitator. This conventional developing device also has one or a plurality of boundary layer preventive plates g provided as near as possible on the surface of the drum d at an opening h of the housing a, i.e., particularly in the vicinity of an opening h' at the rear end in rotating direction of the drum d of the housing a to thus prevent the toner from flying out of the housing together with the entrained air stream above the surface of the drum d. This structure can considerably reduce the leakage of the toner but still leaks a small amount of toner and therefore is not entirely satisfactory.
The applicants have found that the reason why the toner is still leaked in the device shown in FIG. 1 is that there exists no escape of the air circulating partly in the housing a by the preventive plate g or the end of the housing a above the surface of the drum d so that the pressure of the space i surrounded by the housing a and the preventive plate g and the space j formed by the surface of the drum d, surface of the roll b and the preventive plate g is increased with the result that the air containing the toner is ejected from the gap t between the surface of the drum d and the housing.